The Illinois ESR Research Center, established in October of 1985, is a national biomedical resource whose collaborators have cited it in over 103 publications during the first three-and-a-half years of operation. The objective of the IERC is to provide the regional, national, and international biomedical research community with unique and powerful experimental and theoretical capabilities in the area of -Electron Spin Resonance and related techniques. The IERC works towards that objective by focussing on three broad areas of activity: (i) development of state-of- the-art instrumentation to extend the range of experimental possibilities available through paramagnetic resonance investigations, (ii) development of new methodology -- i.e. more powerful theoretical and experimental methods that enhance the precision, sensitivity, and scope of applicability of resonance techniques, and (iii) continual improvement of the IERC as a resource providing the biomedical community with access to fine experimental facilities, expert staff consultation, special services (such as the ESR standard sample project), and up-to-date information, including software developed by the Center, hardware and instrument modifications, workshops, and ESR research and conference news (through the ESR Centers Newsletter). In furtherance of these goals, continuing support is requested for the next five years to allow the IERC to provide the five components of a Research Resource: (1) technological research and development, (2) collaborative research, using the special facilities, methodologies, and expertise of the IERC, with regional, national, and international investigators in the biomedical and related sciences, (3) service activities, (4) several training activities, including a new program to bring in 50 or more graduate or postdoctoral students over the grant period for individualized training programs, and (5) dissemination of information and technology. The technological research and development activities on which IERC will concentrate over the next five-year period are (1) development and perfection of the very-high-frequency W-band ESR spectrometer, the first version of which has become operational; (2) low-frequency (S-band) ESE, and related techniques for hyperfine spectroscopy; (3) computational support for electron spin resonance, (4) ESR imaging developments; and (5) development of low-frequency (L-band in vivo) ESR spectroscopy instrumentation and techniques.